Problem: Nadia is 36 years older than Omar. Nine years ago, Nadia was 5 times as old as Omar. How old is Omar now?
Answer: We can use the given information to write down two equations that describe the ages of Nadia and Omar. Let Nadia's current age be $n$ and Omar's current age be $o$ The information in the first sentence can be expressed in the following equation: $n = o + 36$ Nine years ago, Nadia was $n - 9$ years old, and Omar was $o - 9$ years old. The information in the second sentence can be expressed in the following equation: $n - 9 = 5(o - 9)$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $o$ , it might be easiest to use our first equation for $n$ and substitute it into our second equation. Our first equation is: $n = o + 36$ . Substituting this into our second equation, we get the equation: $(o + 36)$ $-$ $9 = 5(o - 9)$ which combines the information about $o$ from both of our original equations. Simplifying both sides of this equation, we get: $o + 27 = 5 o - 45$ Solving for $o$ , we get: $4 o = 72$ $o = 18$.